In a typical nuclear power plant, operating conditions present unique safety considerations. Foremost of these, is the prevention of any inadvertent release of radiation to the surrounding environment in the unlikely event of equipment malfunction. Additionally, in a system such as a pressurized water reactor (PWR), coolant or water temperatures and pressures can place greater demands on piping as well as other component systems. Typically, a PWR may be operated at temperatures and pressures in the range of about 315.degree. C. (600.degree. F.) and 15.5 MPa (2250 psia), respectively. Such conditions can aggravate high rates of material wear in parts of both primary and secondary piping systems, the latter often referred to as feedwater and extraction steam pipes. Although, not unique to nuclear power plants, such material wear needs to be effectively monitored in order to prevent the possibility of failure of piping systems as a result of material wear. Such failures can cause considerable operational problems, including unscheduled plant shutdowns for emergency pipe replacements or repairs.
Certain phenomena, such as single- and two-phase corrosion/erosion wear of pipes can reduce wall thickness over a given area. This type of wall thinning generally only affects carbon steel pipes which are ferro-magnetic. Experience has shown that such wear is caused by the combined effect of electro-chemical (corrosive) and mechanical (erosive) mechanisms. Under the operating conditions found in secondary piping systems, which are more apt to experience two-phase wear, the steel pipe corrodes, forming a thin layer of iron oxide on the exposed metal surface. Normally, this layer would separate the corrosive environment of the otherwise bare pipe wall from the corroding material and, in the absence of erosive mechanisms, prevent further corrosion of the pipe wall. However, mechanical forces such as fluid turbulence act on the pipe and remove some or all of the oxide layer, thereby removing the protectiveness and allowing the pipe to corrode anew. This cycle of oxide growth and removal continuously wears away the underlying pipe wall material, leading to progressive loss of the pipe's mechanical integrity over extended and lengthy exposure to such wear and can under some conditions ultimately result in the failure of the pipe.
Such wear is found to be dominant in or near bends, tees, and fittings within the piping system. This is because the wear is fundamentally caused by the interactions of the continually flowing liquid in the pipes with the corroded oxide surfaces. In bends, tees and other similar locations, these liquid/material interactions can be more severe and cause relatively more wear. For example, common flow in pipe bends may never be fully developed, which can lead to increased level of local fluid turbulence and greater dissolution of the oxide. Such localization of pipe wall wear can pose a problem with respect to the prediction and correction of pipe wall thinning. Efforts have been made to predict where such pipe wall thinning may tend to occur to help in the deployment of in-service inspection teams to monitor the wear of the carbon steel pipes. One such example of an effort to predict such wear in carbon steel pipes as a function of typical operating conditions within a nuclear power plant, can be found in a Nuclear Regulatory Commission publication entitled "Prediction and Mitigation of Erosive-Corrosive Wear in Secondary Piping Systems of Nuclear Power Plants" by R. G. Keck and P. Griffith, September 1987, referred to as NUREG/CR-A5007.
Even after such areas of localized wear have been identified, it can be difficult to effectively monitor the rate of wear over a given period of time. The rate of wall thinning is typically based on thickness measurement readings taken at different times at the same location and is used in structural integrity life predictions. The use of ultrasonic devices for the measurement of the thickness of a structure to determine its wear is well-known in the art. An example of which is U.S. Pat. No. 4,642,215 issued on Feb. 10, 1987 to Klinvex et al., and assigned to the assignee of the present invention. Moreover, the use of ultrasonic transducers to periodically measure the thickness of a structure over time to monitor that structure so as to prevent its unexpected failure is equally well-known. In nuclear reactor systems such monitoring of vital components of a power plant has increased importance.
In ultrasonic measuring, an ultrasonic beam is sent from a transmitter into the structure to be monitored and a return signal or echo is received by a receiver. The time it takes from transmission to reception is determined, and is converted into a signal indicative of the thickness of the particular structure. Once such a device is calibrated for whatever material is being tested, accurate readings of the structure can be taken.
In measuring secondary piping systems of a nuclear power plant, conventional wall thickness gages require that a grid be painted on the pipe, then measurements are taken point by point. A typical grid may be from 50 to 100 points. This technique can be extremely time consuming and difficult to replicate. A typical pipe may require hundreds of locations to be monitored. With a large number of areas to test, the total number of points to be monitored can be quite numerous, on the order of about one million. In order to perform measurements at a typical location, the following steps typically are carried out. First, insulation must be removed from around the pipe. After a particular location has been identified, the grid must then be painted onto the pipe. However, the grid to be used for testing must first be defined by the technician. With a painted grid, each location must have associated therewith an alphanumeric character in order to identify each particular point. After the grid has thus been identified and defined on the pipe, a point by point analysis, for example, by an ultrasonic transducer, is performed. The readings for each particular point are identified and recorded for future reference.
Such detailed and time consuming preparation for thickness measurement can lead to a high cost for such operations. Also it can be difficult to replicate the exact testing procedure at a later time. During the operation of the plant, it is not uncommon for the painted grid to fade and become undetectable. Thus, for subsequent thickness measurement operations it may be necessary to again identify, define and paint the grid, after first locating the precise location of the previous grid in order to provide for exact replicability of the prior testing. In order to effectively monitor the rate of wall thinning, the subsequent measurements must be taken at the same location. It would be highly desirable to provide a device which would greatly reduce the costly time and procedures now used to provide an indication of pipe wall thickness for a vast number of individual points.